Copy the external string value whose body address and count are on the
parameter stack into the string buffer and push it onto the string stack.
Errors are thrown if the count is larger than MAX_DSCOUNT, if there is not
enough room in string space, even after garbage collection, or if there is
an unterminated string concatenation. The input external string need not
exist as a packed string. "to-string-s-copy"

NOTE: MAX_DSCOUNT is the largest size the count field of a packed string
can hold, e.g., 255 or 64K - 1. It is returned by:
S" /DYNAMIC-STRING" ENVIRONMENT?

WARNING: This word should not be used when the input string is a bound
string because the copy operation may generate a garbage collection which
invalidates its PSA.